1. Field of the Invention
The invention relates to the field of propulsion systems for aircraft and, in particular, to propulsion systems for vertical and/or short take-off aircraft (V/STOL).
2. Description of Related Art
The efficiency of a propulsion system for an aircraft is maximized when the velocity of the exhaust gases equals the velocity of the aircraft in its flight direction at minimum specific fuel consumption. Thus during takeoff, landing, and hovering it is obvious that a helicopter, which provides a small incremental velocity to a large mass of air (low disc loading), is more efficient than a jet aircraft, which provides a large incremental velocity to a small mass of air (high disc loading). However, a helicopter, because of its very large diameter rotor, has a limited forward velocity of less than 200 Knots due to compressibility effects on the rotor blade tips. Thus most V/STOL aircraft are compromises, which either limits the forward velocity of the aircraft (helicopter) or requires oversized engines for vertical flight (jet aircraft) causing a loss in cruise efficiency.
For example, the AV-8A Harrier V/STOL aircraft utilizes a turbofan engine for both hover and cruise propulsion. The turbofan engine was sized to produce adequate thrust for vertical lift in hover, but its correspondingly large frontal area increases the drag of the
aircraft and limits its maximum speed to less than Mach 1 (approximately 580 Knots at sea level). However, the turbofan exhaust is of significantly high velocity and, thus propulsion efficiency is low at cruise velocities because the engine is oversized for this flight mode and hovering, which requires maximum power, for any significant amount of time is avoided because of the high fuel consumption.
In U.S. Pat. No. 4,474,345, "Tandem Fan Series Flow V/STOL Propulsion System" by R. G. Musgrove, a jet engine with a small fan, which is capable of providing supersonic performance, is modified to provide vertical lift. The basic engine fan is split to provide fore and aft fans connected by means of a common drive shaft. The fans are centrally mounted in a duct located within the aircraft along its longitudinal axis. In normal wingborne flight (hereinafter referred to as horizontal flight mode), the fans operate in series with the fan exhaust mixing with the turbine exhaust and exiting through a nozzle located at the rear of the aircraft. In the vertical mode of operation, a diverter is positioned downstream of the forward fan and is movable to a position for diverting the exhaust from the forward fan downward relative to the longitudinal axis of the aircraft, while simultaneously opening an auxiliary inlet for permitting the introduction of air to the aft fan. An aft diverter is located in the nozzle which is also moveable to a position for diverting the exhaust from the aft fan and engine core downward. Thus for vertical flight the diverters are actuated causing the exhaust from both fans and the core engine to be directed downward fore and aft of the center of gravity of the aircraft. However, the tandem fan engine has less thrust in the vertical takeoff and landing mode of operation than it has in the normal flight mode of operation. The thrust is greater in cruise because airflow passes through both fans, and thus the core is supplied with air that is raised to a higher pressure level (supercharged); whereas, in the vertical mode the core engine airflow passes through only the aft fan. Consequently, the tandem fan concept is not an efficient design for a V/STOL aircraft.
Another more efficient approach is to couple a separate large diameter lift fan to the main turbofan by means of a drive shaft. The lift fan is clutched in and powered only during vertical flight modes. In addition, both the fan section and turbine section exhaust are deflected downward to provide lift. Increased performance is obtained because some of the turbofan's power is being used to drive the lift fan, which is more efficient at the low vertical take-off and landing speeds. Such a system can be found in co-pending U.S. patent application Ser. No. 07/521,211 "Propulsion System For A V/STOL Aircraft," filed May 5, 1990. However, as with all the designs discussed above, the propulsion systems are designed primarily for supersonic high-speed flight and modified for V/STOL operation. They are not readily applicable for subsonic aircraft where significant hover time is required.
In U.S. Pat. No. 4,791,783, "Convertible Aircraft Engine" by R. E. Neitzel, a turbofan concept is disclosed for converting almost all the power used by the engine fan to shaft horsepower to drive a helicopter rotor. Guide vanes located on both sides of the outer portion of the engine fan can be actuated to block off air flow through the fan duct while still allowing air flow into the engine core. A gear mounted on the forward end of the fan shaft is coupled to a drive shaft which in turn drives the rotor. Such a system provides maximum efficiency during takeoff and landing and also during normal flight. However, if high-speed flight, (greater than 0.5 Mach) is to be accomplished, the rotor must be either stopped (x-wing concept) or stopped and stowed. The former concept requires an extremely complex computer-controlled pneumatic blowing system that, to this date, has not been successfully developed. The latter system causes a severe weight penalty and requires a complex folding and stowing system. Furthermore, it is difficult to achieve low-observable (LO) characteristics with either design.
The tilt rotor concept, found in the V-22 Osprey aircraft, uses large diameter propellers powered by two cross-shafted turboshaft engines. Its disc loading is higher than a helicopter, but lower than a turbofan and, thus is efficient in the vertical flight modes; however, the large propellers limit the top speed to about 300 Knots at sea level. Again, this is due to compressibility effects on the propeller tips. Furthermore, the large propellers eliminate it as a candidate for missions where a low radar cross-section is required. Tilt pylon-mounted turbofan engines can obtain a higher cruising speed, but lose vertical flight mode efficiency because of the high disc loadings. In addition, pylon-mounted engines of any type, where the fan is visible to radar signals, are also unsuitable for LO missions.
The type of V/STOL aircraft that appears to be most suitable for missions where low radar cross-section is required is one where the entire propulsion system is imbedded in the aircraft wing and/or fuselage. For example, as in a ducted fan-in-wing for the vertical flight mode and turbo-jet or turbofan engines for the horizontal flight modes. The overall concept is rather old, dating at least back to 1914. For example, U.S. Pat. No. 1,130,623 "Flying Machine" by M. L. Mustionen discloses pylon-mounted lift propellers and a pusher propeller mounted in the tail, all powered by a single piston engine. However, with modern V/STOL aircraft, safety requirements dictate the use of multiple engines with cross-shafting to obtain engine-out performance in the vertical flight mode. Examples of this concept can be found in U.S. Pat. Nos. 4,828,203, "Vertical/Short Take-Off And Landing Aircraft" and 4,469,294, "V/STOL Aircraft," both by R. T. Clifton, et al. This aircraft design uses two pylon-mounted ducted propellers for the vertical flight mode and a rear-mounted ducted propeller for the horizontal flight mode. Two engines are mounted in the airframe and "belt drive" a common shaft that is directly connected to the rear mounted propeller. The drive shaft is also connected to a right-angle gearbox which in turn drives the two pylon-mounted ducted lift propellers by means of belt drives. It is apparent that such a combination aircraft design and propulsion system, as configured, does not lend itself to LO missions because of the rear-mounted ducted propeller used for the horizontal flight mode. However, even if it were installed in a proper airframe, it still would not provide the necessary propulsion efficiency and engine-out performance required for any practical aircraft.
The basic problem is that in an aircraft, such as a transport, the ratio of thrust required for takeoff in the vertical flight mode to that required for efficient cruise
in the horizontal flight mode is on the order of 10 to 1. Having multiple engines simply to provide for engine-out capability yields a thrust mismatch between the cruise and vertical flight modes. If the aircraft has only two engines and it requires both engines for a normal takeoff in the vertical flight mode, then each engine alone must be able to provide the total thrust required (in a max power setting for engine-out capability). This means that each of the two engines must be greatly oversized and therefore will yield very poor cruise efficiency. It's either this approach or stay with a single engine, as in the AV-8A Harrier aircraft. To date, no prior design has sufficiently addressed this problem.
Thus it is a primary object of the subject invention to provide a propulsion system for a vertical and/or short take-off and landing aircraft.
It is another primary object of the subject invention to provide a propulsion system for a vertical and/or short take-off and landing aircraft that provides increased propulsive efficiency in the horizontal flight mode.
It is a further object of the subject invention to provide a propulsion system for use in low-observable vertical and/or short take-off and landing aircraft.
It is a still further object of the subject invention to provide a propulsion system for a vertical and/or short take-off and landing aircraft that provides engine-out capability.
It is another object of the subject invention to provide a propulsion system for a vertical and/or short take-off aircraft that provides optimum or near optimum efficiency of the propulsion system in both the vertical and horizontal flight modes.